The Biggest Explosions in the Solar System

The Biggest Explosions in the Solar System

June
12, 2001 -- Astronauts love space walks. Floating weightless
hundreds of kilometers above Earth, the terrain below racing
by at 17,000 mph --no space traveler wants it to end. But it
only takes two words to send one of those brave explorers racing
back to their craft: "Solar flare!"

"Solar flares are the biggest explosions in the solar
system," says Robert Lin of UC Berkeley's Space Science
Lab. "They erupt near sunspots with the force of a hundred
million hydrogen bombs." Astronauts caught spacewalking
during a solar flare or one of their cousins, a coronal mass
ejection, can absorb a radiation dose equivalent to 100 chest
x-rays -- reason enough to dash for shelter.

"One of the most amazing things about solar flares,"
says Brian Dennis of NASA's Goddard Space Flight Center, "is
the efficient way they accelerate subatomic particles to energies
exceeding 109 eV." As much as 50% of the total
explosion energy emerges as electrons and atomic nuclei traveling
at nearly the speed of light. "Flares operate much more
efficiently than any particle accelerator we've been able to
build here on Earth."

"How do flares do that?" he asks. We don't know,
but terrestrial particle physicists would love to find out.

What ignites solar flares? How do they unleash so much energy
so quickly? And is it possible to predict when they will happen?

Such questions have vexed astronomers since 1859 when Lord
Carrington spotted a solar flare for the first time. "I
was [counting sunspots on a projected image of the Sun],"
he recalled, when suddenly "two patches of intensely bright
and white light broke out" near a remarkably large sunspot
group. "Flurried by the surprise," Carrington rushed
from his telescope to call a second witness, but by the time
he returned minutes later the outburst had vanished.

Left:
A 4.2
MB mpeg movie shows a solar flare in action, blasting hot
gas away from the limb of the Sun. [more]

Carrington knew he had glimpsed something enormously powerful,
but what he saw was just the tip of the iceberg. Fast-moving
particles that emerge from flares radiate mostly high-energy
x-rays and gamma-rays. Lower-energy visible light isn't as important.

And therein lies the reason that flares have been able to
guard their secrets for so long. The explosions are brightest
at wavelengths that Earthbound observers can't see with their
eyes. Telescopes are hobbled, too, because our atmosphere is
opaque to x-rays and gamma-rays.

Now a new NASA satellite aims to change all that. The High
Energy Solar Spectroscopic Imager (HESSI for short) will orbit Earth nearly 600 km above our planet's
obscuring atmosphere, where it can record x-ray and gamma-ray
emissions from flares. HESSI, which is slated for launch no earlier
than June 20th, isn't the first spacecraft capable of detecting
such radiation. But it will be the first to capture crisp hard
x-ray and gamma-ray images of the violent explosions.

"The angular resolution of HESSI's hard x-ray images
will be about 2 arcseconds or about as good as you could get
from an optical telescope on the ground." says Lin, the
mission's principal investigator. The gamma-ray images will be
a little less detailed, with resolutions between 7 and 36 arcseconds.
But, Lin notes enthusiastically, "we've never seen any
gamma-ray image of a solar flare before." HESSI's will be
the first.

To
put these numbers into perspective, consider the following: When
a solar flare erupts, it heats a region of the Sun's atmosphere
many Earth-diameters across. (What Carrington saw in 1859 was
the white light "bloom" from such a flare.) HESSI's
hard x-ray images will reveal details only 1700 km wide -- about
the distance between Los Angeles and Seattle.

That's amazing because such high-energy x-rays and gamma-rays
can't be focused; they fly right through conventional lenses.
Instead, HESSI forms images by looking at the Sun through finely-spaced
parallel slats --like microscopic Venetian blinds-- that cast
shadows across onboard radiation detectors. "We'll rotate
the spacecraft every 4 seconds to create a modulation pattern
from the shadows that we can analyze to form an image of the
Sun," explains Lin. The process
is similar to a medical x-ray, except scientists are interested
in the source of the rays (the Sun), not the material that blocks
them (the slats).

HESSI's cameras can make pictures of the entire Sun, but researchers
will be especially interested in sunspots. "That's where
flares erupt -- in the vicinity of sunspots with intense, twisted
magnetic fields," says George Fisher, a colleague of Lin's
at Berkeley. "Twisted magnetic fields are like rubber bands
stretched taut," he explained. "They want to snap back
-- violently. Reconnecting fields are probably the power source
for flares."

At least that's what most solar physicists believe. The problem
is, no one has ever seen it happen. "Before HESSI we couldn't
locate the onset of an eruption with sufficient precision to
make the connection between flares and kinks in the magnetic
field," says Fisher. "I'm dying to know where flare
particles are accelerated, and I think HESSI is finally going
to show us."

HESSI's job is important
fundamental physics, adds NASA's mission scientist Brian Dennis.
Understanding how flares work could teach us how to build better
particle accelerators on Earth and maybe even advance the cause
of fusion power, which also involves superheated gases threaded
by magnetic fields.

Left: Inside the CERN high-energy particle accelerator
in Geneva. Basic research on solar flares might one day improve
such devices on Earth. &COPY; CERN Geneva

HESSI's findings will also shed light on mysterious happenings
far outside our solar system. "Whatever triggers solar flares
could be the same mechanism that blasts jets of particles from
the magnetized accretion disks of black holes and neutron stars,"
says Dennis. "The Sun is comparatively nearby, so it's a
natural laboratory for studying such exotic processes."

Meanwhile most astronauts would be satisfied with simple timely
predictions of garden-variety solar flares, a potential spinoff
of the HESSI mission. If the spacecraft can accomplish that one
thing, space will become a safer place ... for everyone.

Editor's note: Solar flares are closely related to
coronal
mass ejections (CMEs) -- billion-ton clouds of gas that billow
away from the Sun and trigger geomagnetic storms when they strike
Earth's magnetosphere. Scientists once thought flares propelled
CMEs into space, but we've since learned that flares and CMEs
can happen together or separately. Perhaps the two are different
aspects of the same kind of explosion triggered by changing magnetic
fields on the Sun. No one is sure. "One of HESSI's goals
is to understand the relationship between solar flares and CMEs,"
says Dennis. [Listen
to Bob Lin discuss CMEs].

The First Recorded Solar Flare -- Excerpt from: Description of a Singular
Appearance seen in the Sun on September 1, 1859. by Richard C.
Carrington, Monthly Notices of the Royal Astronomical Society,
vol. 20, 13-15, 1860.